英语听力：自然百科 探索世界奥秘之Life On Mars(火星生命)Unit04（在线收听）

These discoveries raised the stakes. Here was a planet similar to our own. Mars might not have cities or civilizations. But it could be home to other smaller life forms like microbes.

Chris McKay is NASA's leading microbe expert. He believes that finding even these minute organisms would be of the utmost importance.

If we find life on Mars, what we're almost certainly gonna find are microbes, little tiny microscopic organisms. You think, well, what's the big deal? Well in fact for most of Earth's history, all the life was on Earth was microbes. And so going to Mars and even finding the smallest, dumbest, littlest bug will still be important because it'll tell us that here in our own back yard life started twice. And so the hard part is possible. The evolution into intelligence and, and humanlike beings is pretty straightforward compared to the question of could it get started.

And there is something about microbes that makes them well suited to surviving on Mars. They're incredibly tough.

They are the organisms also that live in the extremes. They can live in the very high temperature and very high acid and very high salt. They define the limits of life and the potential of life.

But hunting for microbes would not be easy. Mars is 6,700 kilometers wide and it has the same landmass as Earth. Microbes, on the other hand, are tiny. You could fit nearly 10 million on just your fingertip. It would be far harder than finding a needle in a haystack. Scientists had to narrow down their search. So they looked for the one thing that had to be present for life to form: a place where you could find microbes.

The search for life is the search for liquid water. Life on Earth is basically full of water. Life forms are basically little bags of water with little a, a few other ingredients added. But water is the main component. Water is what makes it work.

On Earth, all life is based on water. It's the main constituent of every cell. Because water is basically inert, it's the perfect medium for different types of molecule to flow around, meet and react together. It enables life to form.

microbe: a minute life form; a microorganism, especially a bacterium that causes disease

landmass: a large unbroken area of land

needle in a haystack: an item that is very hard or impossible to locate